1. Field of the Invention
The present invention relates to an electric power steering apparatus so configured as to perform steering assist by transmitting a driving force produced by an electric motor to a steering mechanism.
2. Description of Related Art
Conventionally, an electric power steering apparatus that performs steering assist by transmitting torque produced by an electric motor to a steering mechanism in a vehicle has been employed. The electric motor is driven and controlled on the basis of a target current value determined depending on a steering torque applied to a steering wheel and vehicle speed.
In order to carry out feedback control of the electric motor, a current flowing through the electric motor is detected by a current detection circuit. A motor current value detected by the current detection circuit is inputted to an analog-to-digital (A/D) conversion port in a microcomputer. The microcomputer determines a target current value depending on the steering torque and the vehicle speed, and carries out feedback control of the electric motor such that the motor current value coincides with the target current value.
Although the motor current detection circuit comprises an amplification circuit for amplifying a detection signal, the gain of the amplification circuit is generally fixed. Therefore, the detection resolution of a motor current becomes a fixed value throughout a detection range. For example, the resolution in a case where the A/D conversion resolution of the microcomputer is 10 bits (0˜1023), and the detection range of the motor current is 50 amperes is a fixed value of 50/1023 amperes throughout the detection range.
However, when the steering torque is small, so that the motor current is very small, the steering wheel is considered to be lightly steered. Accordingly, even a very small change in the motor current becomes the change in the steering torque which is easily felt by a driver.
If a high-resolution analog-to-digital (A/D) converter is used, therefore, fine control can be carried out. Correspondingly, it is considered that a steering feeling can be improved. However, the high-resolution A/D converter and a microcomputer equipped therewith are high in cost. Moreover, in a case where the large steering torque is applied so that the motor current is large, the driver applies a large force to the steering wheel. Accordingly, the driver does not mind a very small change in the motor current. Consequently, the use of the high-resolution A/D converter is not so preferable in terms of cost efficiency.
On the other hand, in Japanese Unexamined Patent Publication (KOKAI) No. 2002-46630, A, for example, it is proposed that a motor current is detected using a current detection circuit having two amplification circuits for amplifying a detection signal with different amplification factors. In the prior art, output signals of the two amplification circuits are respectively inputted to two A/D conversion ports in a microcomputer. The microcomputer controls an electric motor by referring to an input signal of the A/D conversion port which receives the signal from the amplification circuit having a high amplification factor when the motor current is small, while controlling the electric motor on the basis of the detection signal received from the A/D conversion port corresponding to the amplification circuit having a low amplification factor when the motor current is large.
Consequently, the motor current can be detected with a high detection resolution in an area where the motor current is very small. Accordingly, a steering feeling in a case where the motor current is very small can be improved.
In the above-mentioned configuration, however, the current detection circuit needs the two amplification circuits. Therefore, the cost of the current detection circuit is increased. Further, the output signals of the two amplification circuits are respectively inputted to the two A/D conversion ports in the microcomputer. Therefore, the microcomputer must be equipped with a lot of A/D conversion ports and the cost thereof is increased accordingly. Further, a plurality of amplification circuits and a plurality of A/D conversion ports are employed. In order to ensure the relative precision of read values, therefore, constant adjustment is required.